In confocal microscopy, a pinhole filter in the confocal plane of the lens eliminates unfocussed light. However, because confocal pinhole filters clip illumination lines projected to the sensor; any shift of that line relative to the pinhole filter will result in a change of the image intensity.
While improving resolution, confocal pinhole filters also increase tool sensitivity to focus and boresight variation due to thermal drifts, vibrations and mechanical repeatability of components. Increased sensitivity affects stability of the tool and eliminating all of the increased sensitivity factors is impractical due to tool complexity.
Consequently, it would be advantageous if an apparatus existed that is suitable for wafer inspection with confocal slit aperture filters and increase stability.